Wednesday, September 29, 2010

No post - vacation!

Regis is in vacation and is not writing a long post on her phone.

Sunday, September 26, 2010

OSPF types of routes

If you look at an OSPF routing table ("sh ip route ospf", for instance), there are a couple of different types of routes that might be displayed.

  • "O" -- An OSPF route from within the same OSPF area.
  • "IA" -- An OSPF route from a different OSPF area.
  • "E1" -- An OSPF route that is a Type 1 external route.
  • "E2" -- an OSPF route that is a Type 2 external route.

An External route is one that is learned from another routing protocol and redistributed into OSPF.  Type E1 is an external route that includes the path metrics from the redistribution other routing protocol and the cost to get to the router that's doing the redistributing.  Type E2 is an external route that only includes the cost from the redistributed routing protocol and does not pay attention to the cost to get to the border router that is doing the redistributing.

Wednesday, September 22, 2010

Open Most Complicated Routing Protocol First

OSPF! Open Shortest Path First.  It's an interior gateway routing protocol - you'd use OSPF within a given AS, for instance.  It has a whole bunch of things that can be tweaked.  Most people (in my experience) use it as their interior routing protocol.

OSPF is based around the idea of a link state diagram -- that is, a diagram of the network connections between all the routers in a given OSPF instance.  When making routing decisions, a given router will figure out what the best ("shortest") next-hop is for a given destination.  If left untweaked, the route metric will be based on the number of hops and the bandwidth of each link (lower-bandwidth links are less preferred).  You can adjust that and manually weight routes to encourage traffic to go over a path that it would otherwise not prefer.

An OSPF-enabled router uses broadcast packets to announce itself and to find out about other OSPF routers in the same collision domain.  If compatible, the routers will establish a neighbor relationship, and exchange route information.  Among neighbors, there's a designated router and a designated backup router.  The designated router is a given router interface that is in charge of sending out link state announcements and letting the other router interfaces know what's up with the links.  It's a specific interface, not a specific individual router -- a router that is a DR on interface 1 might not be the DR for a separate OSPF instance on interface 2.

Sunday, September 19, 2010

Useful tool

Wireshark -- tagline "sniffing the glue that holds the Internet together -- is a network sniffer that uses tcpdump format capture files.  It runs on windows and OSX.  Very useful, with fairly versatile filtering available.  What I particularly like, though, is the ability to follow and display a given TCP session.  It's great for tracking a given web session or something like that.

Wednesday, September 15, 2010

BGP decisionmaking, and a link to the bad science blog about blind prejudice

or "Fucking BGP - how does it work?" context

I'm on a BGP kick because someone was asking me questions about BGP last week and I felt like an idiot when I couldn't remember more than two of the criteria BGP uses to make its routing decisions.

So, here's how BGP makes its routing decisions (and the order in which it evaluates criteria):
  1. Pick the route with the highest "weight" (bgp "weight" is a cisco-specifc thing specified on the local router)
  2. Pick the route with the highest "local pref"
  3. Prefer a route that is locally originated vs remote.
  4. Pick the route with the shortest AS path (You can avoid this by using the "bgp bestpath as-path ignore" command.)
  5. Pick the route with the lowest origin type. (Interior routing protocols such as OSPF are preferred over routes that originate via EBGP)
  6. Pick the route with the lowest "MED", Multi Exit Discriminator.
  7. Prefer EBGP over IBGP.
  8. Prefer the path with the closest (determined by interior routing protocols) next hop.
  9. The router determines if anything MultiPath-related needs to happen.
  10. If we've gotten to this point and still have multiple choices, prefer the oldest route. This helps prevent route flapping.
  11. Prefer the route from the peer with the lowest Router ID. This is sort of like how ospf picks a designated router by "router ID". For BGP, the router ID is the highest IP address on the router, preferably loopback addresses. You can avoid IP address tweaking by manually setting the router ID with the command "bgp router-is".
  12. If you're running an environment with route reflector(s), prefer the shortest cluster list among multiple paths that go to the same originator/router-id.
  13. If not, just pick the path with the lowest neighbor IP address.

Then, RELEASE THE PACKET! (It helps to imagine the TUBEZ of the internet being full of tiny little kraken. Or maybe that's just me.)

Meanwhile, over on the unsurprising gender side of things:
On the Internet, no one knows you're a dog. But if you're a woman, what you wear can change people's perception of your skills, at least for musicians.

Sunday, September 12, 2010


bgp! very important!
here's a bgp overview from apricot 2004, and here's cisco's big page of BGP resources

More detail in later posts.

Thursday, September 9, 2010

An elementary link

yeah, who forgot to click "publish" yesterday? YES, IT WAS ME!

If you're looking to go back to basics, or you're just learning basics, cisco's "Internetworking Technology Handbook" is a pretty good place to start.

It's available online in HTML or for download as a bunch of pdf's.

Sunday, September 5, 2010

no post, holiday

holiday weekend. no post. go have fun.

Wednesday, September 1, 2010

cue the doctor evil voice

Soon, we will see ethernet switches that run of speed of ONE HUNDRED GIG.

Which, you know, that's pretty sexy right there.

Soon, I'm sure, you'll be able to buy 10G switches in blisterpacks at the checkout aisle at microcenter.


About Me

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Regis has worked as a network engineer since 1994 for small companies and for large companies.